This application claims the priority benefit of Taiwan application serial no. 91121105, filed on Sep. 16, 2002.
1. Field of Invention
The present invention generally relates to a light emitting device display technique, and more particularly, to a driving technique of the active matrix organic light emitting diode (AMOLED), so as to increase the driving voltage of the light emitting device as well as the stability with regard to the time passed by.
2. Description of Related Art
Accompanying the development of high technology, video products, especially the digital video or image devices, have become a popular product in our daily life. Within these digital video or image devices, display devices are an important element for displaying the related information. The users can read the information from the display further to control the device operation.
In order to comply with modern life, the size of the video or image device is getting thinner and lighter. The conventional Cathode Ray Tube (CRT) display occupies a large capacity and consumes more electricity. Therefore, complying with photoelectron and semiconductor manufacturing technologies, the panel display device has been developed and has become a common used display product, like the LCD or the active matrix organic light emitting diode display.
LCD technology has developed for several years, so it is hard to have a breakthrough now. However, the active matrix organic light emitting diode (AMOLED) display technology, a newly developed technology, will be main stream of the display device accompanying LCD in the future. The major feature of the AMOLED display is using TFT technique to drive the organic light emitting diode, and the driving IC is installed on the panel directly, so as to fulfill the requirement of being light/thin/short/small in volume and reducing cost. The AMOLED display can be applied on the medium or small size panel in cellular phone, PDA, digital camera and palm game player, portable DVD player and the automobile global positioning system, which can even be implemented in a large size panel like computer and plane TV in the future.
The digital display is characterized by a display screen composed of multiple pixels in a matrix arrangement manner. In order to control individual pixels, a specific pixel is commonly selected via a scanning line and a data line, and an appropriate operating voltage is also provided, so as to display the display information corresponding to this pixel. FIG. 1 schematically shows a sketch map of an AMOLED circuit that drives a corresponding pixel in the prior art. Referring to FIG. 1, the driving circuit comprises transistors 100 and 102. The transistor is such as the thin film transistor (TFT). A gate of the transistor 100 connects to the scanning line and receives a scanning voltage Vscan at an appropriate point of time, and a source of the transistor 100 also receives a digital data voltage Vdata sent from the data line at this point of time. A drain of the transistor 100 connects to a gate of the transistor 102. Generally speaking, the source and the drain of the transistor are swappable. The case shown in the present invention is only exemplified here for description. In addition, a storage capacitor 106 is connected in between the gate of the transistor 102 and a grounded voltage. The drain of the transistor 102 connects to a voltage source VDD, the source of the transistor 102 further connects to an anode of the organic light emitting device 104, and a cathode of the organic light emitting device 104 connects to a corresponding low voltage VSS.
The operation principle of the driving circuit shown in FIG. 1 mentioned above is described as follows. When the gate of the transistor 100 is activated by receiving the Vscan provided by the scanning line, the digital data voltage Vdata is input into the gate of the transistor 100, so as to activate the transistor 102. Meanwhile, the voltage source VDD flows into the organic light emitting device 104 via the transistor 102, and makes it emit the light. The transistor 102 is also generally called a driving device. When the circuit is operating, the scanning line clock Vscan is input into the transistor 100 with a pre-determined frequency, and the time period between its clock pulses is also called a frame. A pre-determined image data block is input into the corresponding pixel during a time period of the frame. When the scanning line clock pulse Vscan activates the transistor 100, the transistor 102 is also subsequently activated by the digital data voltage, and the digital data voltage Vdata is also stored in the storage capacitor 106, so as to maintain the activation of the transistor 102.
Therefore, the conventional organic light emitting device 104 always stays in the activation state in any of the frames. The variance only exists in the fact that the conventional voltage Vdata has different display gray scales in different frames. In other words, the light emitting device of the TFT-AMOLED always makes it stay at the emitting state in the conventional design. Conventionally, such emitting method complies with the image display effect and is able to avoid the picture flicking. In order to have the light emitting device continuously be driven, relatively, the transistor 102 must maintain its activation state.
However, when the light emitting device 104, such as the organic light emitting diode, is operated for a long time period, there is a driving current continuously flowing through the light emitting device 104. Therefore, its characteristic such as the driving voltage VOLED increases over time. Thus, the light emitting state of the light emitting device, such as the variances of the brightness and color, are impacted as shown in FIG. 2. The relationship between the effect caused by the deviation of the driving voltage VOLED and the driving circuit cooperated with the TFT is described hereinafter.
When the organic light emitting device 104 is activated, the TFT driving current ID has a relationship as shown in formula (1)-(4):
xe2x80x83ID=xc2xdk(VSsxe2x88x92Vth)2xe2x80x83xe2x80x83(1)
ID=xc2xdk(VGxe2x88x92VSxe2x88x92Vth)2xe2x80x83xe2x80x83(2)
VS=VOLED+VSSxe2x80x83xe2x80x83(3)
ID=xc2xdk(VGxe2x88x92VOLEDxe2x88x92VSSxe2x88x92Vth)2xe2x80x83xe2x80x83(4)
where k is a TFT characteristic constant, VG=Vdata, and VOLED is the driving voltage of the light emitting device 104. As shown in the formula (1) (4) above, when the driving voltage VOLED increases, since it is activated for a long time, the driving current ID flowing through the organic light emitting device 104 reduces accordingly, thus impacts the light emitting condition of the organic light emitting device 104, and the brightness is also reduced accordingly. The life of the organic light emitting device 104 depends on its light emitting capability. Therefore, the variance of the driving voltage VOLED greatly impacts the organic light emitting device 104.
In addition, similarly, when the transistor 102 is activated for a long time, its threshold voltage Vth increases accordingly. The threshold voltage Vth is the same as the driving voltage VOLED, the current flowing through the light emitting device 102 reduces when the threshold voltage Vth increases. Therefore, the threshold voltage Vth further deteriorates the light emitting quality.
Therefore, the present invention provides a driving circuit for the light emitting device, able to avoid the deviation of the driving voltage VOLED of the light emitting device, and at least maintaining the driving voltage VOLED on a stable value even under a long time operation of displaying image, so as to efficiently improve the display product quality. Furthermore, the threshold voltage Vth can also maintain a stable value without any deviation.
The present invention provides a driving circuit for a light emitting device, suitable for use in an active matrix organic light emitting diode (AMOLED), which has a scanning line to be input with a scanning clock signal, so as to control the driving circuit. The driving circuit includes a driving circuit main part which includes a light emitting device driven by a driving transistor as well as a scan line connection terminal and a data line connection terminal. The scan line connection terminal receives a scanning clock signal. A first transistor has a gate connected to this scan line connection terminal, a source connected to the data line connection terminal, and a drain connected to a gate electrode of the driving transistor. A second transistor has a gate electrode connected to the scan line connection terminal, a source connected to a common voltage, and a drain connected to an anode of the light emitting device. The common voltage has a high voltage level and a low voltage level, alternating by a frequency, wherein the high voltage level of the common voltage is higher than a system low voltage and the low voltage level is smaller than the system low voltage. When the first transistor and the second transistor are simultaneously activated by the scanning clock signal, the data line can be input with an image digital data voltage or a negative turning-off voltage. When the common voltage is at the low voltage level, the negative turning-off voltage is input, so as to turn off the driving transistor and the light emitting device.
As described above, the light emitting device mentioned above comprises an organic light emitting diode.
As described above, the high voltage level of the common voltage mentioned above is 0 V, and the low voltage level mentioned above is a negative voltage.
As described above, the negative turning-off voltage mentioned above is smaller than the low voltage level of the common voltage mentioned above.
As described above, when the first transistor and the second transistor mentioned above are simultaneously activated by the scanning clock signal mentioned above, the data line mentioned above can be input with an image digital data voltage mentioned above to display an image.
As described above, the frequency of the common voltage mentioned above varies in a period of one frame to drive an ON/OFF state of the corresponding multiple scanning lines, so as to achieve a frame inverse operation.
As described above, the frequency of the common voltage mentioned above uses the scanning line as one unit according to the scanning clock signal mentioned above, so as to achieve a line inverse operation.